This invention relates to vanadyl pyrophosphate oxidation catalyst precursors comprising vanadium phosphate esters and to a process of converting the precursors to vanadyl pyrophosphate oxidation catalysts by thermal decomposition.
Maleic anhydride is used as a raw material for numerous products, including agricultural chemicals, paints, paper sizing, food additives and synthetic resins. To fill the high demand for this valuable chemical, a variety of commercial processes have been developed for its production, the most successful of which involves the vapor phase oxidation of n-butane to maleic anhydride in the presence of a vanadyl pyrophosphate ("VPO") catalyst. Since the development of this method in the 1970's, research has continued to improve the reaction conditions and, particularly, the VPO catalysts.
A review of the improvements made in this technology is given by G. J. Hutchings, in Applied Catalysis, Elsevier Science Publishers B. V. Amsterdam, 72(1991), pages 1-31. The preferred method of preparation of VPO catalysts is the hydrochloric acid digestion of V.sub.2 O.sub.5 and H.sub.3 PO.sub.4 in either an aqueous solvent, as described, for example, in U.S. Pat. No. 3,985,775, or non aqueous solvent, such as methanol, tetrahydrofuran (THF) or isobutanol, followed by solvent removal to give what is termed the catalyst precursor, vanadium hydrogen phosphate, VO(HOPO.sub.3).multidot.(H.sub.2 O)0.5. The precursor is then activated by heating, as described, for example, in U.S. Pat. No. 3,864,280 and U.S. Pat. No. 4,043,943. Other methods are described in U.S. Pat. No. 4,132,670 and U.S. Pat. No. 4,442,226.
Johnson et al. (Inorg Chem. 23, 3842 (1984); J Am. Chem. Soc. 111, 381 (1989)) have described a series of compounds given by the formula VO(RPO.sub.3).multidot.H.sub.2 O R'OH, where R' represent alkyl groups from alcohol molecules and R represent alkyl groups attached to phosphorus atoms. These compounds are described as vanadyl alkylphosphonates since the alkyl groups, R, are bound directly to the trivalent phosphorus. Alkyl groups expressed as C.sub.n H.sub.2n +1 for n=2-9, 14 and 18 are disclosed. While these compounds are layered, only those with n&lt;3 and just H.sub.2 O in the interlayer space have structures which are analogous to VO(HOPO.sub.3).multidot.(H.sub.2 O)0.5. Thermogravimetric results of Huan et al. (Materials Chemistry and Physics 35, 199 (1993)) and Guliants et al. (Chem. Mater. 7, 8! 1493 (1995)) showed that complete thermal decomposition of these phases in air requires temperatures &gt;500.degree. C. and -375-475.degree. C., respectively. Johnson et al. did not comment on the decomposition products. Guliants et al. did disclose that these compounds could be converted to VPO at temperatures lower than the vanadium hydrogen phosphate, VO((HOPO.sub.3).multidot.(H.sub.2 O)0.5.
U.S. Pat. No. 5,401,707 describes a series of compounds with the general formula, VO(HOPO.sub.3).multidot.(H.sub.2 O)0.5 ({C.sub.n H.sub.2n +}-X).sub.z in which n has a value of from 0 to 20, z has a value of from about 1.5 to about 1.9; and X is an oxygen or nitrogen containing functional group. These phases consist of the known VOHOP.sub.3 .multidot.(H.sub.2 O)0.5 intercalated with a compound such as an alkyl amine. The conversion of these precursors to (VO).sub.2 P.sub.2 O.sub.7 at temperature of at least 400.degree. C. is disclosed.
Alagna et al. (Mat. Res. Bull. 22, 691 (1987)) have described the synthesis of VOHOPO.sub.3 .multidot.(H.sub.2 O).sub.1.25 .multidot.(EtOH)0.5. This material is reported to have a basal plane spacing of 1.358 nm and a structure consisting of vanadyl hydrogen phosphate, VO(OHPO.sub.3).multidot.(H.sub.2 O)0.5 whose layers are intercalated with ethanol molecules. The compound decomposes, non-topotactically, to an amorphous substance at .about.400.degree. C.